1. Field of the Invention
This invention relates generally to implantable devices, such as an expandable intraluminial prosthesis, one example of which includes a stent. More particularly, the invention is directed to an apparatus and method for coating a prosthesis.
2. Description of the Related Art
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress the atherosclerotic plaque of the lesion against the inner wall of the artery to dilate the lumen. The balloon is deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, an expandable intraluminal prosthesis, one example of which includes a stent, is implanted in the lumen to maintain the vascular patency. Stents are scaffoldings, usually cylindrical or tubular in shape, which function to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed for insertion through small cavities via small catheters, and expanded to a larger diameter once at the desired location. Examples in patent literature disclosing stents which have been successfully applied in PTCA procedures include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
To treat the damaged vasculature tissue and assist prevention of thrombosis and restenosis, there is a need for administrating therapeutic substances to the treatment site. For example, anticoagulants, antiplatelets and cytostatic agents are commonly used to prevent thrombosis of the coronary lumen, to inhibit development of restenosis, and to reduce post-angioplasty proliferation of the vascular tissue, respectively. To provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered at a specific site in comparison to larger overall dosages that are applied systemically. Local delivery produces fewer side effects and achieves more effective results.
One commonly applied technique for the local delivery of a drug is through the use of a polymeric carrier coated onto the surface of a stent, as disclosed in U.S. Pat. No. 5,464,650 issued to Berg et al. Berg disclosed applying to a stent body a solution which included a specified solvent, a specified polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend. The solvent was allowed to evaporate, leaving on the stent surface a coating of the polymer and the therapeutic substance impregnated in the polymer. As indicated by Berg, stents were immersed in the solution 12 to 15 times or sprayed 20 times.
The immersion method of coating a stent, also called dip-coating, entails submerging the entire stent, or an entire section of the stent, in a polymer solution. Similarly, spray-coating requires enveloping the entire stent, or an entire section of the stent, in a large cloud of polymeric material. One disadvantage of dip-coating and spray-coating methods is the inability to control the exact geometrical pattern of coating on the stent or section of the stent. Another shortcoming of both dip- and spray-coating is the possibility of forming web-like defects by build-up of excess polymeric material within the radii of the stent. Web-like defects are most prevalent in stents having tight patterns, for example coronary stents, such that the radii are very small.
Another disadvantage of both dip-coating and spray-coating stems from a low-viscosity requirement for the polymer solution in which the stent is dipped or with which the stent is sprayed. A low viscosity solution can only be achieved by using a low molecular weight polymer or by using a very low concentration of polymer in the polymer solution. Thus, both dip-coating and spray-coating methods have imposed limitations in type and concentration of applied polymers.
Other commonly applied techniques for coating a stent with a polymeric material include sputtering and gas phase polymerization. Sputtering typically involves placing a polymeric coating material target in an environment, and applying energy to the environment that hits the target and causes emission of polymeric material from the target. The polymeric emissions deposit onto the stent, forming a coating. Similarly, gas phase polymerization typically entails applying energy to a monomer in the gas phase within a system set up such that the polymer formed is attracted to a stent, thereby creating a coating around the stent.
Sputtering and gas phase polymerization have similar shortcomings. Like the dip-coating and spray-coating techniques, the sputtering and gas phase polymerization techniques do not allow control of the geometrical pattern in which the stent will be coated and are quite limited in the selection of polymers that can be employed. In addition, coating a stent with a polymer and a drug at the same time via sputtering or gas phase polymerization has not been demonstrated to be effective and risks degradation of the drug. Moreover, techniques for applying a polymeric coating by sputtering or gas phase polymerization and later incorporating a drug into the applied polymeric coating are limited.
Accordingly, it is desirable to provide an improved method of applying a polymeric coating to a prosthesis. Specifically, it is desirable to provide a method of applying a polymeric coating to a prosthesis which enables control over the geometrical pattern in which a prosthesis is coated, reduces the incidence of web-like defects due to excess build-up of polymeric material, broadens the field of both the types and the concentrations of polymers which may be used to coat a prosthesis, and allows a prosthesis to be coated with a polymer and a drug at the same time.